Lockstep systems are fault-detection CPU systems that redundantly execute the same set of operations at the same time in parallel. The redundancy allows error detection. In particular, the output from lockstep operations can be compared to determine if there has been a fault if there are at least two systems (dual modular redundancy).
To run in lockstep, each redundant system is set up to progress from one well-defined state to the next well-defined state. When a new set of inputs reaches the system, each redundant system processes the new set of inputs, generates new outputs and updates a respective state. This set of changes (new inputs, new outputs and new state) is considered to define that step, and is treated as an atomic transaction. In other words, either all of the set of changes happen, or none of set of changes happen, and not something in between.